Semiconductor device

ABSTRACT

A nonvolatile memory is provided. A semiconductor device (a nonvolatile memory) has a circuit configuration similar to that of a general SRAM. By providing a transistor whose off-state current is small between a stored data holding portion and a power supply line of the SRAM, leakage of electric charge from the stored data holding portion is prevented. As the transistor whose off-state current is small provided for preventing leakage of electric charge from the stored data holding portion, a transistor including an oxide semiconductor film is preferably used. Such a configuration can also be applied to a shift register, whereby a shift register with low power consumption can be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device. In thisspecification, a semiconductor device refers to a semiconductor elementitself or a device including a semiconductor element. Examples of such asemiconductor element include, for example, a thin film transistor.Accordingly, a semiconductor device also includes a device such as aliquid crystal display device in its category.

2. Description of the Related Art

As one kind of a volatile memory, a static random access memory (SRAM)is widely known.

Meanwhile, in recent years, a metal oxide having semiconductorcharacteristics (hereinafter referred to as an oxide semiconductor) hasattracted attention. An oxide semiconductor can be applied to atransistor (see Patent Documents 1 and 2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

It is an object of an embodiment of the present invention to provide asemiconductor device including a nonvolatile memory.

An embodiment of the present invention is a semiconductor device (anonvolatile memory) having a circuit configuration similar to that of ageneral SRAM. By providing a transistor whose off-state current is smallbetween a stored data holding portion and a power supply potential lineof the SRAM, leakage of electric charge from the stored data holdingportion is prevented. As the transistor whose off-state current issmall, for example, a transistor in which a channel formation region isformed using an oxide semiconductor layer may be used.

Note that in the drawings, in order to express a very small off-statecurrent of a transistor in which a channel formation region is formedusing an oxide semiconductor layer, part of the transistor is indicatedby a dashed line.

In accordance with an embodiment of the present invention, it ispossible to provide a semiconductor device including a nonvolatilememory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are circuit diagrams each illustrating an example of anonvolatile memory according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a shift register to which the nonvolatilememory in FIG. 1A is applied.

FIG. 3 is a diagram illustrating operation of the shift register in FIG.2.

FIGS. 4A and 4B are circuit diagrams each illustrating an example of anonvolatile memory according to an embodiment of the present invention.

FIG. 5 is a circuit diagram of a shift register to which the nonvolatilememory in FIG. 4A is applied.

FIG. 6 is a cross-sectional schematic view of an applicable transistor.

FIGS. 7A to 7D illustrate a method for manufacturing the transistor inFIG. 6.

FIGS. 8A to 8C are diagrams each illustrating a crystal structure of anoxide material.

FIGS. 9A to 9C are diagrams illustrating a crystal structure of an oxidematerial.

FIGS. 10A to 10C are diagrams illustrating a crystal structure of anoxide material.

FIGS. 11A to 11C are diagrams illustrating a crystal structure of anoxide material.

FIGS. 12A to 12E are equations for calculating the mobility.

FIG. 13 is a graph showing a relation between gate voltage andfield-effect mobility.

FIGS. 14A to 14C are graphs each showing a relation between gate voltageand drain current.

FIGS. 15A to 15C are graphs each showing a relation between gate voltageand drain current.

FIGS. 16A to 16C are graphs each showing a relation between gate voltageand drain current.

FIGS. 17A to 17C are graphs each showing characteristics of atransistor.

FIGS. 18A and 18B are graphs each showing characteristics of atransistor.

FIGS. 19A and 19B are graphs each showing characteristics of atransistor.

FIG. 20 is a graph showing temperature dependence of off-state currentof a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description and it iseasily understood by those skilled in the art that modes and detailsthereof can be variously changed without departing from the spirit andthe scope of the present invention. Accordingly, the present inventionshould not be construed as being limited to the description of theembodiments below.

Embodiment 1

First, examples of a configuration of a nonvolatile memory according toan embodiment of the present invention will be described.

FIG. 1A illustrates an example of a configuration of a nonvolatilememory according to an embodiment of the present invention. Thenonvolatile memory illustrated in FIG. 1A includes first to eighthtransistors. The first to fourth transistors are electrically connectedto each other in series in that order. The fifth to eighth transistorsare electrically connected to each other in series in that order. One ofa source and a drain of the first transistor 101 is electricallyconnected to a high power supply potential line. One of a source and adrain of the fourth transistor 104 is electrically connected to a lowpower supply potential line. One of a source and a drain of the fifthtransistor 105 is electrically connected to the high power supplypotential line. One of a source and a drain of the eighth transistor 108is electrically connected to the low power supply potential line. A gateof the first transistor 101 and a gate of the fourth transistor 104 areelectrically connected to a third terminal 130. The third terminal 130is electrically connected between one of a source and a drain of thesixth transistor 106 and one of a source and a drain of the seventhtransistor 107. A gate of the fifth transistor 105 and a gate of theeighth transistor 108 are electrically connected to a second terminal120. The second terminal 120 is electrically connected between one of asource and a drain of the second transistor 102 and one of a source anda drain of the third transistor 103. A gate of the second transistor102, a gate of the third transistor 103, a gate of the sixth transistor106, and a gate of the seventh transistor 107 are electrically connectedto a first terminal 110.

In other words, the nonvolatile memory illustrated in FIG. 1A has thefollowing connections. One of the source and the drain of the firsttransistor 101 is electrically connected to the high power supplypotential line. The other of the source and the drain of the firsttransistor 101 is electrically connected to one of the source and thedrain of the second transistor 102. The other of the source and thedrain of the second transistor 102 is electrically connected to one ofthe source and the drain of the third transistor 103. The other of thesource and the drain of the third transistor 103 is electricallyconnected to one of the source and the drain of the fourth transistor104. The other of the source and the drain of the fourth transistor 104is electrically connected to the low power supply potential line. One ofthe source and the drain of the fifth transistor 105 is electricallyconnected to the high power supply potential line. The other of thesource and the drain of the fifth transistor 105 is electricallyconnected to one of the source and the drain of the sixth transistor106. The other of the source and the drain of the sixth transistor 106is electrically connected to one of the source and the drain of theseventh transistor 107. The other of the source and the drain of theseventh transistor 107 is electrically connected to one of the sourceand the drain of the eighth transistor 108. The other of the source andthe drain of the eighth transistor 108 is electrically connected to thelow power supply potential line. The gate of the first transistor 101and the gate of the fourth transistor 104 are electrically connected tothe third terminal 130. The third terminal 130 is electrically connectedbetween the sixth transistor 106 and the seventh transistor 107. Thegate of the fifth transistor 105 and the gate of the eighth transistor108 are electrically connected to the second terminal 120. The secondterminal 120 is electrically connected between the second transistor 102and the third transistor 103. The gate of the second transistor 102, thegate of the third transistor 103, the gate of the sixth transistor 106,and the gate of the seventh transistor 107 are electrically connected tothe first terminal 110.

The first transistor 101 and the fifth transistor 105 are each ap-channel transistor, and the fourth transistor 104 and the eighthtransistor 108 are each an n-channel transistor. The second transistor102, the third transistor 103, the sixth transistor 106, and the seventhtransistor 107 are each an n-channel transistor in which a channelformation region is formed using an oxide semiconductor layer.

In the drawings, the high power supply potential line is denoted by Vddand the low power supply potential line is denoted by Vss (this alsoapplies to the description below). It is preferable that the potentialof the high power supply potential line be the highest potentialsupplied from a power supply, and the potential of the low power supplypotential line be a ground potential.

Next, operation of the nonvolatile memory having the configurationillustrated in FIG. 1A is described. In the nonvolatile memory havingthe configuration illustrated in FIG. 1A, a low potential is supplied tothe second terminal 120, so that the fifth transistor 105 is turned onand the eighth transistor 108 is turned off. At this time, a highpotential is supplied to the first terminal 110, so that the sixthtransistor 106 and the seventh transistor 107 are turned on. Thus, thethird terminal 130 is electrically connected to the high power supplypotential line Vdd via the fifth transistor 105 and the sixth transistor106 and thus has a high potential.

Since the third terminal 130 has a high potential, the first transistor101 is turned off and the fourth transistor 104 is turned on. At thistime, the first terminal 110 has a high potential as described above;therefore, the second transistor 102 and the third transistor 103 areturned on. Accordingly, the second terminal 120 is electricallyconnected to the low power supply potential line Vss via the thirdtransistor 103 and the fourth transistor 104 and thus maintains a lowpotential.

After that, a low potential is supplied to the first terminal 110, sothat the second terminal 120 and the third terminal 130 are in anelectrically floating state. The second transistor 102, the thirdtransistor 103, the sixth transistor 106, and the seventh transistor 107are each a transistor in which a channel formation region is formedusing an oxide semiconductor layer and has a small off-state current;therefore, electric charges of the second terminal 120 and the thirdterminal 130 are held. Accordingly, even when the nonvolatile memory ispowered off and the high power supply potential line has a low potential(e.g., Vss), the potentials of the second terminal 120 and the thirdterminal 130 are held. Consequently, when the nonvolatile memory ispowered on again and a high potential is supplied to the first terminal110, the operation can be started again before the state where the highpower supply potential line has a low potential (the nonvolatile memoryis powered off).

As described above, when the nonvolatile memory illustrated in FIG. 1Ais powered off, the first terminal 110 has a low potential, whereby thesecond transistor 102, the third transistor 103, the sixth transistor106, and the seventh transistor 107 are turned off. Since the secondtransistor 102, the third transistor 103, the sixth transistor 106, andthe seventh transistor 107 are each a transistor in which a channelformation region is formed using an oxide semiconductor layer, a regionserving as an electric charge holding portion (denoted by a heavy linein FIG. 1A) can hold electric charge. Accordingly, even when thenonvolatile memory is temporarily powered off, the nonvolatile memorycan operate correctly because electric charge is held in the electriccharge holding portion. In addition, stored data can be held even whenpower is not supplied to the power supply potential line (even when thenonvolatile memory is powered off) at a time other than writing andreading operations, so that power consumption can be reduced.

The configuration of the nonvolatile memory illustrated in FIG. 1A isone example, and the configuration of the nonvolatile memory of thepresent invention is not limited thereto. FIG. 1B illustrates an exampleof a configuration of a nonvolatile memory according to an embodiment ofthe present invention which is different from that in FIG. 1A.

FIG. 1B illustrates an example of a configuration of a nonvolatilememory according to an embodiment of the present invention. In order toobtain the nonvolatile memory illustrated in FIG. 1B, the configurationin FIG. 1A is changed as follows. The first transistor 101 and thesecond transistor 102 are replaced with each other, the third transistor103 and the fourth transistor 104 are replaced with each other, thefifth transistor 105 and the sixth transistor 106 are replaced with eachother, and the seventh transistor 107 and the eighth transistor 108 arereplaced with each other. Connections of gates of these transistors arethe same as those in the nonvolatile memory in FIG. 1A.

The first transistor 101 and the fifth transistor 105 are each ap-channel transistor, and the fourth transistor 104 and the eighthtransistor 108 are each an n-channel transistor. The second transistor102, the third transistor 103, the sixth transistor 106, and the seventhtransistor 107 are each an n-channel transistor in which a channelformation region is formed using an oxide semiconductor layer.

When the nonvolatile memory illustrated in FIG. 1B is powered off (whena low potential is supplied to the high power supply potential lineVdd), the first terminal 110 has a low potential, whereby the secondtransistor 102, the third transistor 103, the sixth transistor 106, andthe seventh transistor 107 are turned off. Since the second transistor102, the third transistor 103, the sixth transistor 106, and the seventhtransistor 107 are each a transistor in which a channel formation regionis formed using an oxide semiconductor layer and thus has a smalloff-state current, a region serving as an electric charge holdingportion (denoted by a heavy line in FIG. 1B (part of electric charge isalso held in a region denoted by a heavy dashed line)) can hold electriccharge. Accordingly, even when the nonvolatile memory is temporarilypowered off, the nonvolatile memory can operate correctly becauseelectric charge is held in the electric charge holding portion. Inaddition, stored data can be held even when power is not supplied to thepower supply potential line (even when the nonvolatile memory is poweredoff) at a time other than writing and reading operations, so that powerconsumption can be reduced.

The nonvolatile memories illustrated in FIGS. 1A and 1B can be appliedto a shift register. FIG. 2 illustrates a circuit configuration of ashift register including the plurality of nonvolatile memoriesillustrated in FIG. 1A. A rectangular region denoted by a dashed linecorresponds to the nonvolatile memory illustrated in FIG. 1A. FIG. 3 isa timing chart showing operation of the shift register in FIG. 2.

It is also possible to form a shift register by providing the pluralityof nonvolatile memories illustrated in FIG. 1B.

In FIG. 2, CLK denotes a line to which a clock signal is input, iCLKdenotes a line to which an inversion signal of the clock signal isinput, and D denotes a data input signal line.

It is preferable in FIG. 2 that each of the transistors surrounded bydashed circles be a transistor in which a channel formation region isformed using an oxide semiconductor layer. A transistor in which theclock signal CLK is input to a gate and a transistor in which theinversion signal iCLK of the clock signal is input to a gate are eachpreferably a transistor in which a channel formation region is formedusing an oxide semiconductor layer.

In FIG. 3, when the nonvolatile memory is powered off at a time t1, thehigh power supply potential line Vdd has a low potential, and thepotential is held even when the nonvolatile memory is powered off. Asshown in FIG. 3, a low potential is supplied to a control signal line φimmediately before the time t1. Then, a low potential is also suppliedto the high power supply potential line Vdd, and the potentials are heldeven when the nonvolatile memory is powered off. Thus, by supplying ahigh potential to the high power supply potential line Vdd immediatelyafter a time t2 and then supplying a high potential to the controlsignal line φ, the operation can be started again before the state wherethe nonvolatile memory is powered off.

The configuration of the nonvolatile memory according to an embodimentof the present invention is not limited to the configurationsillustrated in FIGS. 1A and 1B. For example, in the configuration inFIG. 1A, the third transistor 103 may also serve as the fourthtransistor 104.

FIG. 4A illustrates an example of a configuration of a nonvolatilememory according to an embodiment of the present invention. Thenonvolatile memory illustrated in FIG. 4A has the following connections.One of a source and a drain of a first transistor 101 is electricallyconnected to a high power supply potential line. The other of the sourceand the drain of the first transistor 101 is electrically connected toone of a source and a drain of a second transistor 102. The other of thesource and the drain of the second transistor 102 is electricallyconnected to one of a source and a drain of a third transistor 103. Theother of the source and the drain of the third transistor 103 iselectrically connected to a low power supply potential line. One of asource and a drain of a fifth transistor 105 is electrically connectedto the high power supply potential line. The other of the source and thedrain of the fifth transistor 105 is electrically connected to one of asource and a drain of a sixth transistor 106. The other of the sourceand the drain of the sixth transistor 106 is electrically connected toone of a source and a drain of a seventh transistor 107. The other ofthe source and the drain of the seventh transistor 107 is electricallyconnected to the low power supply potential line. A gate of the firsttransistor 101 and a gate of the third transistor 103 are electricallyconnected to a third terminal 130. The third terminal 130 iselectrically connected between the other of the source and the drain ofthe sixth transistor 106 and one of the source and the drain of theseventh transistor 107. A gate of the fifth transistor 105 and a gate ofthe seventh transistor 107 are electrically connected to a secondterminal 120. The second terminal 120 is electrically connected betweenthe other of the source and the drain of the second transistor 102 andone of the source and the drain of the third transistor 103. A gate ofthe second transistor 102 and a gate of the sixth transistor 106 areelectrically connected to a first terminal 110.

The first transistor 101 and the fifth transistor 105 are each ap-channel transistor. The second transistor 102, the third transistor103, the sixth transistor 106, and the seventh transistor 107 are eachan n-channel transistor in which a channel formation region is formedusing an oxide semiconductor layer.

Instead of the configuration in FIG. 4A, a configuration in FIG. 4B maybe employed. A nonvolatile memory illustrated in FIG. 4B has thefollowing connections. One of a source and a drain of a secondtransistor 102 is electrically connected to a high power supplypotential line. The other of the source and the drain of the secondtransistor 102 is electrically connected to one of a source and a drainof a first transistor 101. The other of the source and the drain of thefirst transistor 101 is electrically connected to one of a source and adrain of a third transistor 103. The other of the source and the drainof the third transistor 103 is electrically connected to a low powersupply potential line. One of a source and a drain of a sixth transistor106 is electrically connected to the high power supply potential line.The other of the source and the drain of the sixth transistor 106 iselectrically connected to one of a source and a drain of a fifthtransistor 105. The other of the source and the drain of the fifthtransistor 105 is electrically connected to one of a source and a drainof a seventh transistor 107. The other of the source and the drain ofthe seventh transistor 107 is electrically connected to the low powersupply potential line. A gate of the first transistor 101 and a gate ofthe third transistor 103 are electrically connected to a third terminal130. The third terminal 130 is electrically connected between the otherof the source and the drain of the fifth transistor 105 and one of thesource and the drain of the seventh transistor 107. A gate of the fifthtransistor 105 and a gate of the seventh transistor 107 are electricallyconnected to a second terminal 120. The second terminal 120 iselectrically connected between the other of the source and the drain ofthe first transistor 101 and one of the source and the drain of thethird transistor 103. A gate of the second transistor 102 and a gate ofthe sixth transistor 106 are electrically connected to a first terminal110.

The configurations in FIGS. 4A and 4B are preferable to theconfigurations in FIGS. 1A and 1B because the number of the elements canbe smaller.

The nonvolatile memories illustrated in FIGS. 4A and 4B can be appliedto a shift register. FIG. 5 illustrates a circuit configuration of ashift register including the plurality of nonvolatile memoriesillustrated in FIG. 4A. A rectangular region denoted by a dashed linecorresponds to the nonvolatile memory illustrated in FIG. 4A. Theoperation of the shift register in FIG. 5 is the same as that in FIG. 2;thus, the description thereof is omitted here. It is also possible toform a shift register by providing the plurality of nonvolatile memoriesillustrated in FIG. 4B.

It is also preferable in FIG. 5 that each of the transistors surroundedby dashed circles be a transistor in which a channel formation region isformed using an oxide semiconductor layer. Both a transistor in which aclock signal CLK is input to a gate and a transistor in which aninversion signal iCLK of the clock signal is input to a gate may eachpreferably be a transistor in which a channel formation region is formedusing an oxide semiconductor layer.

The nonvolatile memory according to an embodiment of the presentinvention has been described above. Note that the p-channel transistorand the n-channel transistor in the nonvolatile memory are not limitedto particular transistors, and a variety of transistors can be used.Thus, the p-channel transistor and the n-channel transistor may each bea transistor formed using polycrystalline silicon or a transistor formedusing a silicon on insulator (SOI) substrate.

Next, a transistor which can be used for the nonvolatile memory will bedescribed. As the transistor in which a channel formation region isformed using an oxide semiconductor layer, a transistor including ametal oxide having semiconductor characteristics can be used. As atransistor other than the transistor in which a channel formation regionis formed using an oxide semiconductor layer, a transistor formed usinga semiconductor substrate can be used.

FIG. 6 schematically illustrates examples of cross-sectional structuresof transistors which can be used for the nonvolatile memory. In thenonvolatile memory illustrated in FIG. 6, a transistor in which achannel formation region is formed using an oxide semiconductor layer isformed over a transistor formed using a semiconductor substrate. Thetransistor formed using a semiconductor substrate can be a p-channeltransistor or an n-channel transistor.

For example, the first transistor 101 and the fifth transistor 105 inFIG. 1A may each be the p-channel transistor formed using asemiconductor substrate. For example, the fourth transistor 104 and theeighth transistor 108 in FIG. 1A may each be the n-channel transistorformed using a semiconductor substrate. For example, the secondtransistor 102, the third transistor 103, the sixth transistor 106, andthe seventh transistor 107 in FIG. 1A may each be the transistor inwhich a channel formation region is formed using an oxide semiconductorlayer.

The p-channel transistor and the n-channel transistor formed using asemiconductor substrate may be formed by a known method. After thep-channel transistor and/or the n-channel transistor formed using asemiconductor substrate is formed, the transistor in which a channelformation region is formed using an oxide semiconductor layer is formedthereover. That is, by using a semiconductor substrate 200 provided withthe p-channel transistor and/or the n-channel transistor as a substrate,the transistor in which a channel formation region is formed using anoxide semiconductor layer is formed over the semiconductor substrate200.

The semiconductor substrate 200 provided with the p-channel transistorand/or the n-channel transistor includes high-concentration impurityregions 201 serving as a source region and a drain region,low-concentration impurity regions 202, a gate insulating film 203, agate electrode 204, and an interlayer insulating film 205 (FIG. 6).

A transistor 210 in which a channel formation region is formed using anoxide semiconductor layer includes an oxide semiconductor layer 211 overthe semiconductor substrate 200 provided with the p-channel transistorand/or the n-channel transistor, a source electrode 212 a and a drainelectrode 212 b which are separated from each other and in contact withthe oxide semiconductor layer 211, a gate insulating film 213 over atleast a channel formation region of the oxide semiconductor layer 211,and a gate electrode 214 overlapping with the oxide semiconductor layer211 over the gate insulating film 213 (FIG. 7D).

The interlayer insulating film 205 also serves as a base insulating filmof the oxide semiconductor layer 211.

The interlayer insulating film 205 contains oxygen at least in itssurface and is formed using an insulating oxide from which part ofoxygen is released by heat treatment. As an insulating oxide from whichpart of oxygen is released by heat treatment, a material containing moreoxygen than that in the stoichiometric proportion is preferably used.This is because oxygen can be diffused to the oxide semiconductor filmin contact with the interlayer insulating film 205 by the heattreatment.

As an insulating oxide containing more oxygen than that in thestoichiometric proportion, silicon oxide represented by SiO_(x) wherex>2 can be given, for example. However, an embodiment of the presentinvention is not limited thereto, and the interlayer insulating film 205may be formed using silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum oxynitride, gallium oxide, hafnium oxide, yttrium oxide,or the like.

The interlayer insulating film 205 may be a stack of a plurality offilms. The interlayer insulating film 205 may have a stacked-layerstructure in which a silicon oxide film is formed over a silicon nitridefilm, for example.

In an insulating oxide which contains more oxygen than that in thestoichiometric proportion, part of oxygen is easily released by heattreatment. The amount of released oxygen (the value converted into thatof oxygen atoms) obtained by TDS analysis when part of oxygen is easilyreleased by heat treatment is greater than or equal to 1.0×10¹⁸atoms/cm³, preferably greater than or equal to 1.0×10²⁰ atoms/cm³, morepreferably greater than or equal to 3.0×10²⁰ atoms/cm³.

Here, how to perform TDS analysis is described. The release amount of agas in the TDS analysis is proportional to a time integral value of theion intensity. Thus, from the time integral value of the ion intensityof an oxide and a reference value of a standard sample, the releaseamount of the gas can be calculated. The reference value of a standardsample refers to the ratio of the density of a predetermined atomcontained in a sample (standard sample) to the time integral value ofthe ion intensity.

For example, by using the ion intensity of a silicon wafer containing apredetermined density of hydrogen (standard sample) and the ionintensity of an oxide, the release amount (N_(O2)) of oxygen molecules(O₂) from the oxide can be obtained by the formula:N_(O2)=N_(H2)/S_(H2)×S_(O2)×α.

N_(H2) is a value obtained by conversion of hydrogen molecules (H₂)released from the standard sample into density. S_(H2) is the timeintegral value of the ion intensity of hydrogen molecules (H₂) of thestandard sample. In other words, the reference value of the standardsample is N_(H2)/S_(H2). S_(O2) is the time integral value of the ionintensity of oxygen molecules (O₂) of the oxide. α is a coefficientwhich influences the ion intensity. Refer to Japanese Published PatentApplication No. H6-275697 for details of the formula.

Note that the release amount of oxygen obtained by TDS analysis (thevalue converted into that of oxygen atoms) is measured with the use of asilicon wafer containing hydrogen atoms at 1×10¹⁶ atoms/cm³ as thestandard sample, by using a thermal desorption spectrometer,EMD-WA1000S/W manufactured by ESCO, Ltd.

Note that in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the coefficient α includes the ionization rate of the oxygenmolecules, the amount of the released oxygen atoms can also becalculated through the evaluation of the amount of the released oxygenmolecules.

In addition, N_(O2) is the amount of released oxygen molecules (O₂).Thus, the amount of released oxygen converted into oxygen atoms is twicethe amount of released oxygen molecules (O₂).

The interlayer insulating film 205 may be formed by a sputtering method,a CVD method, or the like, and is preferably formed by a sputteringmethod. In the case where a silicon oxide film is formed as theinterlayer insulating film 205, a quartz (preferably, synthesizedquartz) target may be used as a target, and an argon gas may be used asa sputtering gas. Alternatively, a silicon target and a gas containingoxygen may be used as a target and a sputtering gas, respectively. As agas containing oxygen, a mixed gas of an argon gas and an oxygen gas maybe used or only an oxygen gas may be used.

After the interlayer insulating film 205 is formed, first heat treatmentis performed before the oxide semiconductor film to be the oxidesemiconductor layer 211 is formed. The first heat treatment is performedin order to remove water and hydrogen contained in the interlayerinsulating film 205. The temperature of the first heat treatment may behigher than or equal to 400° C. and lower than a temperature at whichthe semiconductor substrate 200 provided with the p-channel transistorand/or the n-channel transistor is changed in quality or shape,preferably higher than or equal to 400° C. and lower than or equal to750° C., that is, lower than the temperature of second heat treatmentperformed later.

Then, the second heat treatment is performed after the oxidesemiconductor film is formed. Through the second heat treatment, oxygenis supplied from the interlayer insulating film 205 to the oxidesemiconductor film. The timing of the second heat treatment is notlimited to this timing, and the second heat treatment may be performedafter the oxide semiconductor layer 211 is formed by processing theoxide semiconductor film.

Note that it is preferable that in the second heat treatment, hydrogen,water, a hydroxyl group, hydride, and the like be not contained in anitrogen gas or a rare gas such as a helium gas, a neon gas, or an argongas. Alternatively, the purity of a nitrogen gas or a rare gas such as ahelium gas, a neon gas, or an argon gas that is introduced into a heattreatment apparatus is preferably higher than or equal to 6N (99.9999%),more preferably higher than or equal to 7N (99.99999%) (that is, theimpurity concentration be lower than or equal to 1 ppm, preferably lowerthan or equal to 0.1 ppm).

Further, the oxide semiconductor film or the oxide semiconductor layer211 may be crystallized to be a microcrystalline layer or apolycrystalline layer depending on a condition of the second heattreatment or a material of the oxide semiconductor film or the oxidesemiconductor layer 211. For example, the oxide semiconductor film orthe oxide semiconductor layer 211 may become a microcrystalline oxidesemiconductor layer having a degree of crystallization of higher than orequal to 90%, or higher than or equal to 80%. Further, depending on thecondition of the second heat treatment or the material of the oxidesemiconductor film or the oxide semiconductor layer 211, the oxidesemiconductor film or the oxide semiconductor layer 211 may become anamorphous oxide semiconductor layer containing no crystalline component.The oxide semiconductor film or the oxide semiconductor layer 211 mayalso become an oxide semiconductor layer in which a microcrystallinepart (with a grain diameter of 1 nm to 20 nm, typically 2 nm to 4 nm) ismixed in an amorphous oxide semiconductor layer.

In the second heat treatment, the interlayer insulating film 205 servesas an oxygen supply source. The interlayer insulating film 205 ispreferably a stack of a layer serving as an oxygen supply source and alayer protecting the layer serving as an oxygen supply source so thatoxygen is not released in the first heat treatment. The layer serving asan oxygen supply source is preferably formed using silicon oxide, andthe layer protecting the layer serving as an oxygen supply source ispreferably formed using aluminum oxide.

It is preferable that the average surface roughness (R_(a)) of theinterlayer insulating film 205 over which the oxide semiconductor filmis formed be greater than or equal to 0.1 nm and less than 0.5 nm. Thisis because crystal orientations can be aligned when the oxidesemiconductor film is a crystalline film.

Note that here, the average surface roughness (R_(a)) is obtained byexpanding the center line average roughness (R_(a)) which is defined byJISB0601:2001 (ISO 4287:1997) into three dimensions so that R_(a) can beapplied to a measurement surface. The average surface roughness (R_(a))is an average value of the absolute values of deviations from thereference surface to the specific surface.

Here, the center line average roughness (R_(a)) is shown by thefollowing formula (1) assuming that a portion having a measurementlength L is picked up from a roughness curve in the direction of thecenter line, the direction of the center line of the roughness curve ofthe picked portion is represented by an X-axis, the direction oflongitudinal magnification (direction perpendicular to the X-axis) isrepresented by a Y-axis, and the roughness curve is expressed as Y=F(X).

$\begin{matrix}{R_{a} = {\frac{1}{L}{\int_{0}^{L}{{{F(X)}}\ {\mathbb{d}X}}}}} & (1)\end{matrix}$

When the measurement surface which is a surface represented bymeasurement data is expressed as Z=F(X,Y), the average surface roughness(R_(a)) is an average value of the absolute values of deviations fromthe reference surface to the specific surface and is shown by thefollowing formula (2).

$\begin{matrix}{R_{a} = {\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}{\int_{X_{1}}^{X_{2}}{{{{F\left( {X,Y} \right)} - Z_{0}}}\ {\mathbb{d}X}\ {\mathbb{d}Y}}}}}} & (2)\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a rectangular region which is surrounded by fourpoints represented by the coordinates (X₁, Y₁), (X₁, Y₂), (X₂, Y₁), and(X₂, Y₂). S₀ represents the area of the specific surface when thespecific surface is flat ideally.

In addition, the reference surface refers to a surface parallel to anX-Y surface at the average height of the specific surface. In short,when the average value of the height of the specific surface is denotedby Z₀, the height of the reference surface is also denoted by Z₀.

Chemical mechanical polishing (CMP) treatment may be performed so thatthe average surface roughness of the interlayer insulating film 205 canbe greater than or equal to 0.1 nm and less than 0.5 nm CMP treatmentmay be performed before formation of the oxide semiconductor film but ispreferably performed before the first heat treatment.

The CMP treatment may be performed at least once. When the CMP treatmentis performed plural times, first polishing is preferably performed witha high polishing rate followed by final polishing with a low polishingrate.

Instead of the CMP treatment, dry etching or the like may be performedin order to planarize the interlayer insulating film 205. As an etchinggas, a chlorine-based gas such as a chlorine gas, a boron chloride gas,a silicon chloride gas, or a carbon tetrachloride gas, a fluorine-basedgas such as a carbon tetrafluoride gas, a sulfur fluoride gas, or anitrogen fluoride gas, or the like can be used as appropriate.

Instead of the CMP treatment, plasma treatment or the like may beperformed in order to planarize the interlayer insulating film 205. Arare gas may be used in the plasma treatment. In the plasma treatment, asurface to be processed is irradiated with ions of an inert gas, andminute projections and depressions on the surface to be processed areplanarized by a sputtering effect. Such plasma treatment is alsoreferred to as reverse sputtering.

In order to planarize the interlayer insulating film 205, at least oneof the above treatments may be performed. For example, only reversesputtering may be performed. Alternatively, dry etching may be performedafter the CMP treatment is performed. Note that it is preferable thatdry etching or reverse sputtering be used so that water can be preventedfrom entering the interlayer insulating film 205 over which the oxidesemiconductor film is formed. In particular, in the case whereplanarization treatment is performed after the second heat treatment,dry etching or reverse sputtering is preferably used.

The oxide semiconductor layer 211 may be, for example, selectivelyformed in such a manner that an oxide semiconductor film is formed, anetching mask is formed over the oxide semiconductor film, and etching isperformed. Alternatively, an ink-jet method or the like may be used.

The oxide semiconductor film may be formed using the following: afour-component metal oxide such as an In—Sn—Ga—Zn—O-based metal oxide, athree-component metal oxide such as an In—Ga—Zn—O-based metal oxide(also referred to as IGZO), an In—Sn—Zn—O-based metal oxide, anIn—Al—Zn—O-based metal oxide, a Sn—Ga—Zn—O-based metal oxide, anAl—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metal oxide, or atwo-component metal oxide such as an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, a Sn—Mg—O-based metal oxide, an In—Mg—O-based metal oxide,or an In—Ga—O-based metal oxide. Alternatively, a one-component metaloxide such as an In—O-based metal oxide, a Sn—O-based metal oxide, or aZn—O-based metal oxide may be used. Note that an n-component metal oxideincludes n kinds of metals. Here, for example, an In—Ga—Zn—O-based metaloxide means an oxide containing indium (In), gallium (Ga), and zinc(Zn), and there is no particular limitation on the composition ratiothereof. The In—Ga—Zn—O-based metal oxide may contain an element otherthan In, Ga, and Zn.

Note that it is preferable that oxygen be contained in the metal oxidein excess of the amount in the stoichiometric proportion. When oxygen iscontained in excess, generation of carriers due to oxygen vacancies inthe oxide semiconductor film which is formed can be suppressed.

Note that for example, in the case where the oxide semiconductor film isformed using an In—Zn—O-based metal oxide, a target has an atomic ratiowhere In/Zn is 0.5 to 50, preferably 1 to 20, more preferably 1.5 to 15.When the amount of Zn in the atomic ratio is in the above preferredrange, the field-effect mobility of the transistor can be improved.Here, when the atomic ratio of the compound is In:Zn:O=X:Y:Z, it ispreferable to satisfy the relation of Z>1.5X+Y so that oxygen iscontained in excess.

Note that the energy gap of a metal oxide which can be applied to theoxide semiconductor film is preferably greater than or equal to 2 eV,more preferably greater than or equal to 2.5 eV, still more preferablygreater than or equal to 3 eV. The off-state current of the transistorcan be reduced by using a metal oxide having a wide band gap asdescribed above.

Note that the oxide semiconductor film contains hydrogen. The hydrogenmay be contained in the oxide semiconductor film in the form of ahydrogen molecule, water, a hydroxyl group, or other hydride in somecases, in addition to the form of a hydrogen atom. It is preferable thatthe amount of hydrogen contained in the oxide semiconductor film be assmall as possible.

Note that the concentrations of an alkali metal and an alkaline earthmetal in the oxide semiconductor film are preferably low, and theseconcentrations are preferably lower than or equal to 1×10¹⁸ atoms/cm³,more preferably lower than or equal to 2×10¹⁶ atoms/cm³. This is becausean alkali metal and an alkaline earth metal are bonded to an oxidesemiconductor to generate carriers in some cases, in which case theoff-state current of the transistor is increased.

Note that there is no particular limitation on the formation method andthe thickness of the oxide semiconductor film and they can be determinedin consideration of the size or the like of the transistor to bemanufactured. As an example of a method for forming the oxidesemiconductor film, a sputtering method, a coating method, a printingmethod, a pulsed laser deposition method, or the like can be given. Thethickness of the oxide semiconductor film is preferably greater than orequal to 3 nm and less than or equal to 50 nm.

Here, as a preferable example, the oxide semiconductor film is formed bya sputtering method using an In—Ga—Zn—O-based metal oxide target. A raregas (for example, an argon gas), an oxygen gas, or a mixed gas of a raregas and an oxygen gas may be used as a sputtering gas.

It is preferable that a high-purity gas from which hydrogen, water, ahydroxyl group, hydride, and the like is removed be used as a sputteringgas for the formation of the oxide semiconductor film. In order to keepthe high purity of a sputtering gas, preferably, a gas attached to theinner wall or the like of a treatment chamber is removed, and thesemiconductor substrate 200 provided with the p-channel transistorand/or the n-channel transistor is subjected to heat treatment beforethe oxide semiconductor film is formed. In addition, a high-puritysputtering gas may be introduced to the treatment chamber. In that case,the purity of an argon gas may be higher than or equal to 9N(99.9999999%), the dew point thereof may be −121° C., the concentrationof water may be 0.1 ppb, and the concentration of hydrogen may be 0.5ppb. The purity of an oxygen gas may be higher than or equal to 8N(99.999999%), the dew point thereof may be −112° C., the concentrationof water may be 1 ppb, and the concentration of hydrogen may be 1 ppb.When the oxide semiconductor film is formed while the semiconductorsubstrate 200 provided with the p-channel transistor and/or then-channel transistor is kept at high temperature, the concentration ofimpurities contained in the oxide semiconductor film can be reduced.Here, the semiconductor substrate 200 provided with the p-channeltransistor and/or the n-channel transistor may be kept at a temperatureof higher than or equal to 100° C. and lower than or equal to 600° C.,preferably higher than or equal to 200° C. and lower than or equal to400° C.

Note that the oxide semiconductor film may have an amorphous structureor a crystalline structure. In the case where the oxide semiconductorfilm has a crystalline structure, a c-axis aligned crystalline (CAAC)oxide semiconductor film is preferably used. When the oxidesemiconductor film is a CAAC oxide semiconductor film, the reliabilityof the transistor can be increased.

Note that a CAAC oxide semiconductor film means an oxide semiconductorfilm including a crystal which has c-axis alignment and a triangular orhexagonal atomic arrangement when seen from the direction of an a-bplane, a surface, or an interface. In the crystal, metal atoms arearranged in a layered manner, or metal atoms and oxygen atoms arearranged in a layered manner along the c-axis, and the direction of thea-axis or the b-axis is varied in the a-b plane (or the surface, or atthe interface) (the crystal rotates around the c-axis).

Note that in a broad sense, a CAAC oxide semiconductor film means anoxide semiconductor film which is non-single-crystal and includes aphase which has a triangular, hexagonal, regular triangular, or regularhexagonal atomic arrangement when seen from the direction perpendicularto the a-b plane and in which metal atoms are arranged in a layeredmanner or metal atoms and oxygen atoms are arranged in a layered mannerwhen seen from the direction perpendicular to the c-axis.

The CAAC oxide semiconductor film is not a single crystal, but this doesnot mean that the CAAC oxide semiconductor film is composed of only anamorphous component. Although the CAAC oxide semiconductor film includesa crystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

In the case where oxygen is included in the CAAC oxide semiconductorfilm, nitrogen may be substituted for part of oxygen. The c-axes ofindividual crystalline portions included in the CAAC oxide semiconductorfilm may be aligned in one direction (e.g., a direction perpendicular toa surface of a substrate over which the CAAC oxide semiconductor film isformed, or a surface, a film surface, or an interface of the CAAC oxidesemiconductor film). Alternatively, normals of the a-b planes ofindividual crystalline portions included in the CAAC oxide semiconductorfilm may be aligned in one direction (e.g., a direction perpendicular tothe surface of the substrate over which the CAAC oxide semiconductorfilm is formed, or a surface, a film surface, or an interface of theCAAC oxide semiconductor film).

Note that the CAAC oxide semiconductor film may be a conductor, asemiconductor, or an insulator depending on its composition or the like.Further, the CAAC oxide semiconductor film may transmit or not transmitvisible light depending on its composition or the like.

As an example of such a CAAC oxide semiconductor film, there is amaterial which is formed into a film shape and has a triangular orhexagonal atomic arrangement when observed from the directionperpendicular to a surface of the film, a surface of a substrate, or aninterface and in which metal atoms are arranged in a layered manner ormetal atoms and oxygen atoms (or nitrogen atoms) are arranged in alayered manner when a cross section of the film is observed.

The crystal structure of such a CAAC oxide semiconductor film will bedescribed in detail. In the description below, in principle, a verticaldirection is a c-axis direction and a plane perpendicular to the c-axisdirection is an a-b plane in FIGS. 8A to 8C and FIGS. 9A and 9B. In thecase where the expressions “an upper half” and “a lower half” are simplyused, they refer to an upper half above the a-b plane and a lower halfbelow the a-b plane (an upper half and a lower half with respect to thea-b plane). Furthermore, in FIGS. 8A to 8C, O surrounded by a circlerepresents a tetracoordianate O atom and O surrounded by a double circlerepresents a tricoordinate O atom.

FIG. 8A illustrates a structure having one hexacoordinate metal atom M_1and six tetracoordinate oxygen (hereinafter tetracoordinate O) atomsproximate to the metal atom M_1. Such a structure in which one metalatom and oxygen atoms proximate to the metal atom are only illustratedis called a subunit here. The structure in FIG. 8A is actually anoctahedral structure, but is illustrated as a planar structure forsimplicity. Note that three tetracoordinate O atoms exist in each of anupper half and a lower half in FIG. 8A.

FIG. 8B illustrates a structure having one pentacoordinate metal atomM_2, three tricoordinate oxygen (hereinafter tricoordinate O) atomsproximate to the metal atom M_2, and two tetrocoordinate O atomsproximate to the metal atom M_2. All the tricoordinate O atoms exist onthe a-b plane. One tetracoordinate O atom exists in each of an upperhalf and a lower half in FIG. 8B.

FIG. 8C illustrates a structure having one tetracoordinate metal atomM_3 and four tetracoordinate O atoms proximate to the metal atom M_3. InFIG. 8C, one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through a tetracoordinate O atom. Specifically, ametal atom is bonded to another metal atom through tetracoordinate Oatoms, the number of which is four in total. For example, in the casewhere the hexacoordinate metal atom M_1 is bonded through threetetracoordinate O atoms in the lower half, it is bonded to thepentacoordinate metal atom M_2 through a teteracoordinate O atom in theupper half of the pentacoordinate metal atom M_2, the pentacoordinatemetal atom M_2 through a teteracoordinate O atom in the lower half ofthe pentacoordinate metal atom M_2, or the tetracoordinate metal atomM_3 through a tetracoordinate O atom in the upper half of thetetracoordinate metal atom M_3.

Besides, subunits are bonded to each other so that the total electriccharge in the layered structure is 0 to form a group.

FIG. 9A illustrates a model of a group included in a layered structureof an In—Sn—Zn—O-based material. A structure in which one metal atom andoxygen atoms proximate to the metal atom are only illustrated is calleda subunit here. A plurality of subunits form a group. A plurality ofgroups form one cycle as illustrated in FIG. 9B which is called a unit.Note that FIG. 9C illustrates an atomic arrangement in the case wherethe layered structure in FIG. 9B is observed from the directionperpendicular to the film surface, substrate surface, or interface.

In FIG. 9A, a tricoordinate O atom is omitted for simplicity, and atetracoordinate O atom is illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, inFIG. 9A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 9A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the group included in the layered structure of an In—Sn—Zn—O-basedmaterial in FIG. 9A, in the order starting from the top, a Sn atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half is bonded to an In atom proximate to one tetracoordinate Oatom in each of an upper half and a lower half, the In atom is bonded toa Zn atom proximate to three tetracoordinate O atoms in an upper half,the Zn atom is bonded through one tetracoordinate O atom in its lowerhalf to an In atom proximate to three tetracoordinate O atoms in each ofan upper half and a lower half, the In atom is bonded to a Zn atomproximate to one tetracoordinate O atom in an upper half, the Zn atom isbonded through three tetracoordinate O atoms in its lower half to a Znatom, and the Zn atom is bonded through one tetracoordinate O atom inits lower half to a Sn atom. A plurality of such groups are bonded toform one unit that corresponds to one cycle.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Thus, electriccharge of a subunit including Sn is +1. Accordingly, electric charge of−1, which cancels electric charge of +1, is needed to form a layeredstructure including a Sn atom. As a structure having electric charge of−1, a structure in which two subunits including Zn atoms are bonded asshown in FIG. 9A can be given. For example, when one subunit including aSn atom is combined with one structure in which two subunits includingZn atoms are bonded, electric charge is canceled, whereby the totalelectric charge in the layered structure can be 0.

In addition, an In atom can have either five ligands or six ligands.When a structure in which one cycle illustrated in FIG. 9B is repeatedis formed, In—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Notethat a layered structure of the obtained In—Sn—Zn—O-based material canbe expressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above description also applies to the case of using thefour-component metal oxide, the three-component metal oxide, thetwo-component metal oxide, or the one-component metal oxide describedabove.

FIG. 10A illustrates an example of a model of a first group included ina layered structure of an In—Ga—Zn—O-based material.

In the first group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 10A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Znatom is bonded to a Ga atom proximate to three tetracoordinate O atomsin an upper half, the Ga atom is bonded through one tetracoordinate Oatom in its lower half to an In atom proximate to three tetracoordinateO atoms in each of an upper half and a lower half. A plurality of firstgroups, three first groups here, are bonded to form one unit thatcorresponds to one cycle.

FIG. 10B illustrates a structure of one cycle including the plurality offirst groups. Note that FIG. 10C illustrates an atomic arrangement inthe case where the layered structure in FIG. 10B is observed from thedirection perpendicular to the film surface, substrate surface, orinterface.

The group included in the layered structure of the In—Ga—Zn—O-basedmaterial is not limited to the first group illustrated in FIG. 10A and acombination of other subunits may be employed. For example, FIG. 11Aillustrates a second group included in another layered structure of anIn—Ga—Zn—O-based material and FIG. 11B illustrates a structure includinga plurality of second groups that corresponds to one cycle. Note thatFIG. 11C illustrates an atomic arrangement in the case where the layeredstructure in FIG. 11B is observed from the direction perpendicular tothe film surface, substrate surface, or interface.

FIG. 11A illustrates a model of the second group as another example ofthe layered structure of the In—Ga—Zn—O-based material.

In the second group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 11A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Ga atom proximate to onetetracoordinate O atom in an upper half, the Ga atom is bonded throughthree tetracoordinate O atoms its lower half to a Zn atom proximate toone tetracoordinate O atom in each of an upper half and a lower half,and the Zn atom is bonded through one tetracoordinate O atom in itslower half to an In atom proximate to three tetracoordinate O atoms ineach of an upper half and a lower half. A plurality of second groups,three second groups here, are bonded to form one unit that correspondsto one cycle.

Electric charge of a (hexacoordinate or pentacoordinate) In atom,electric charge of a (tetracoordinate) Zn atom, and electric charge of a(pentacoordinate) Ga atom are +3, +2, and +3, respectively. Thus,electric charge of a subunit including In, Zn, and Ga is 0. Thus, thetotal electric charge of a layered structure having a combination ofsuch subunits is always 0.

The group included in the layered structure of the In—Ga—Zn—O-basedmaterial is not limited to the first group illustrated in FIGS. 10A to10C and the second group illustrated in FIGS. 11A to 11C, and a grouphaving various combinations of atoms can be used. For example, asillustrated in FIGS. 10A to 10C and FIGS. 11A to 11C, it is sufficientthat the oxide semiconductor film has a structure which has c-axisalignment and a triangle or hexagonal atomic arrangement when seen fromthe direction of an a-b plane, a surface, or an interface and in whichmetal atoms are arranged in a layered manner along the c-axis or metalatoms and oxygen atoms are arranged in a layered manner along thec-axis. In addition, one unit is not limited to the structure includingthree first groups or three second groups and can have variouscombinations.

By using the above-described CAAC oxide semiconductor film, thefield-effect mobility of the transistor can be higher than or equal to30 cm²/Vs.

Here, a method for forming the CAAC oxide semiconductor film isdescribed.

First, an oxide semiconductor film is formed by a sputtering method, amolecular beam epitaxy method, an atomic layer deposition method, apulsed laser deposition method, or the like. Note that by forming anoxide semiconductor film while keeping the substrate 200 provided withthe p-channel transistor and/or the n-channel transistor at hightemperature, the ratio of a crystalline portion to an amorphous portioncan be high. At this time, the temperature of the substrate 200 providedwith the p-channel transistor and/or the n-channel transistor may be,for example, higher than or equal to 150° C. and lower than or equal to450° C., preferably higher than or equal to 200° C. and lower than orequal to 350° C.

Here, the formed oxide semiconductor film may be subjected to heattreatment. By the heat treatment, the ratio of a crystalline portion toan amorphous portion can be high. The temperature of the semiconductorsubstrate 200 provided with the p-channel transistor and/or then-channel transistor at the heat treatment is, for example, higher thanor equal to 200° C. and lower than a temperature at which thesemiconductor substrate 200 provided with the p-channel transistorand/or the n-channel transistor is changed in quality or shape,preferably higher than or equal to 250° C. and lower than or equal to450° C. The time for the heat treatment may be longer than or equal to 3minutes, and preferably shorter than or equal to 24 hours. This isbecause when the time for the heat treatment is increased, the ratio ofa crystalline portion to an amorphous portion can be high; however, theproductivity is decreased. Note that the heat treatment may be performedin an oxidation atmosphere or an inert atmosphere; however, there is nolimitation thereon. This heat treatment may also be performed under areduced pressure.

The oxidation atmosphere is an atmosphere containing an oxidizing gas.As an example of the oxidizing gas, for example, oxygen, ozone, andnitrous oxide can be given. It is preferable that components (e.g.,water and hydrogen) which are not preferably contained in the oxidesemiconductor film be removed from the oxidation atmosphere as much aspossible. For example, the purity of oxygen, ozone, or nitrous oxide ishigher than or equal to 8N (99.999999%), preferably higher than or equalto 9N (99.9999999%).

The oxidation atmosphere may contain an inert gas such as a rare gas.Note that the oxidation atmosphere contains an oxidizing gas at aconcentration of higher than or equal to 10 ppm.

Note that an inert atmosphere contains an inert gas (a nitrogen gas, arare gas, or the like) and contains a reactive gas such as an oxidizinggas at a concentration of lower than 10 ppm.

Note that a rapid thermal anneal (RTA) apparatus may be used for all theheat treatments. With the use of an RTA apparatus, only in a short time,the heat treatments can be performed at high temperature. Thus, theoxide semiconductor film in which the ratio of a crystalline portion toan amorphous portion is high can be formed and a decrease inproductivity can be suppressed.

However, the apparatus used for all the heat treatments is not limitedto an RTA apparatus; for example, an apparatus provided with a unit thatheats an object to be processed by thermal conduction or thermalradiation from a resistance heater or the like may be used. For example,an electric furnace or a rapid thermal anneal (RTA) apparatus such as agas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermal anneal(LRTA) apparatus can be given as the heat treatment apparatus used forall the heat treatments. An LRTA apparatus is an apparatus for heatingan object to be processed by radiation of light (an electromagneticwave) emitted from a lamp such as a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or ahigh pressure mercury lamp. A GRTA apparatus is an apparatus for heatingan object to be processed using a high-temperature gas as a heatingmedium. Here, the temperature of the high-temperature gas is preferablyhigher than the heating temperature of the object to be processed.

With the use of an In—Ga—Zn—O-based metal oxide in which the nitrogenconcentration is higher than or equal to 1×10¹⁷ atoms/cm³ and lower thanor equal to 5×10¹⁹ atoms/cm³, a metal oxide film having a c-axis-alignedhexagonal crystal structure is formed and one or more layers containingGa and Zn are provided between two layers of the In—O crystal planes(crystal planes containing indium and oxygen).

In order to form an In—Sn—Zn—O-based metal oxide, for example, a targetin which the atomic ratio of In:Sn:Zn is 1:2:2, 2:1:3, 1:1:1, or20:45:35 may be used.

As described above, the CAAC oxide semiconductor film can be formed.

The CAAC oxide semiconductor film has high orderliness of a bond betweenmetal and oxygen as compared to an oxide semiconductor film having anamorphous structure. In other words, in the case of an oxidesemiconductor film having an amorphous structure, the number of oxygenatoms coordinated around a metal atom may vary according to the kind ofan adjacent metal. In contrast, in the case of the CAAC oxidesemiconductor film, the number of oxygen atoms coordinated around ametal atom is substantially the same. Thus, an oxygen vacancy is hardlyobserved even at a microscopic level, and electric charge transfer andinstability of electric conductivity due to hydrogen atoms (includinghydrogen ions), alkali metal atoms, or the like can be suppressed.

Thus, when a transistor in which a channel formation region is formedusing a CAAC oxide semiconductor is formed, the amount of change in thethreshold voltage of the transistor before and after light irradiationor a bias-temperature stress (BT) test performed on the transistor canbe suppressed, and the transistor can have stable electriccharacteristics.

Next, a second etching mask is formed over the oxide semiconductor filmand etching is performed, whereby the oxide semiconductor layer 211 isformed (FIG. 7A).

Then, the source electrode 212 a and the drain electrode 212 b which areseparated from each other and in contact with the oxide semiconductorlayer 211 are formed (FIG. 7B).

The source electrode 212 a and the drain electrode 212 b may beselectively formed in such a manner that, for example, a conductive film(e.g., a metal film or a silicon film to which an impurity elementimparting one conductivity type is added) is formed by a sputteringmethod, an etching mask is formed over the conductive film, and etchingis performed. Alternatively, an ink-jet method may be used. Note thatthe conductive film to be the source electrode 212 a and the drainelectrode 212 b may be formed with a single-layer structure or astructure in which a plurality of layers are stacked. The conductivefilm may be formed to have a three-layer structure in which an Al layeris sandwiched between Ti layers, for example. Note that the sourceelectrode 212 a and the drain electrode 212 b also serve as signallines.

Then, the gate insulating film 213 is formed over at least the channelformation region of the oxide semiconductor layer 211 (FIG. 7C).

As the gate insulating film 213, a film may be formed by, for example, asputtering method using an insulating material (e.g., silicon nitride,silicon nitride oxide, silicon oxynitride, or silicon oxide). Note thatthe gate insulating film 213 may be formed to have either a single-layerstructure or a stacked-layer structure including a plurality of layers.A stacked-layer structure of two layers in which a silicon oxynitridelayer is stacked over a silicon nitride layer is employed here, forexample. Note that in the case where the gate insulating film 213 isformed by a sputtering method, hydrogen and moisture can be preventedfrom entering the oxide semiconductor layer 211. The gate insulatingfilm 213 is preferably an insulating oxide film, in which case oxygen issupplied to the oxide semiconductor layer to fill the oxygen vacancies.

Further, “silicon nitride oxide” contains more nitrogen than oxygen.Note that “silicon oxynitride” contains more oxygen than nitrogen.

Note that the processing of the oxide semiconductor film may beperformed by dry etching. For example, a chlorine gas or a mixed gas ofa boron trichloride gas and a chlorine gas may be used as an etching gasused for the dry etching. However, there is no limitation thereon; wetetching may be used or another method capable of processing the oxidesemiconductor film may be used.

The gate insulating film 213 contains oxygen at least in a portion incontact with the oxide semiconductor layer 211 and is preferably formedusing an insulating oxide from which part of oxygen is released byheating. In other words, the materials given as the material of theinterlayer insulating film 205 are preferably used. When the portion ofthe gate insulating film 213 which is in contact with the oxidesemiconductor layer 211 is formed using silicon oxide, oxygen can bediffused to the oxide semiconductor layer 211 and a reduction in theresistance of the transistor can be prevented.

Note that the gate insulating film 213 may be formed using a high-kmaterial such as hafnium silicate (HfSiO_(x)), hafnium silicate to whichnitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z)), hafnium oxide, yttrium oxide, orlanthanum oxide, whereby gate leakage current can be reduced. Here, gateleakage current refers to leakage current which flows between a gateelectrode and a source or drain electrode. Further, a layer formed usingthe high-k material and a layer formed using silicon oxide, siliconoxynitride, silicon nitride, silicon nitride oxide, aluminum oxide,aluminum oxynitride, or gallium oxide may be stacked. Note that even inthe case where the gate insulating film 213 has a stacked-layerstructure, the portion in contact with the oxide semiconductor layer 211is preferably formed using an insulating oxide.

The gate insulating film 213 may be formed by a sputtering method. Thethickness of the gate insulating film 213 is greater than or equal to 1nm and less than or equal to 300 nm, preferably greater than or equal to5 nm and less than or equal to 50 nm. When the thickness of the gateinsulating film 213 is greater than or equal to 5 nm, gate leakagecurrent can be particularly reduced.

Further, third heat treatment (preferably at a temperature of higherthan or equal to 200° C. and lower than or equal to 400° C., forexample, at a temperature of higher than or equal to 250° C. and lowerthan or equal to 350° C.) may be performed in an inert gas atmosphere oran oxygen gas atmosphere. Hydrogen or moisture contained in the oxidesemiconductor layer 211 can be diffused to the gate insulating film bythe third heat treatment. Through the third heat treatment, oxygen canbe supplied from the gate insulating film 213 to the oxide semiconductorlayer 211.

The third heat treatment may be performed not only after the gateinsulating film 213 is formed over the oxide semiconductor layer 211 butalso after the gate electrode 214 or a conductive film to be the gateelectrode 214 is formed.

The concentration of hydrogen in the oxide semiconductor layer 211 ispreferably lower than or equal to 5.0×10¹⁹ atoms/cm³, more preferablylower than or equal to 5.0×10¹⁸ atoms/cm³.

Then, a conductive film is formed over the gate insulating film 213, athird etching mask is formed over the conductive film, and etching isperformed, whereby the gate electrode 214 is formed (FIG. 7D). Note thatthe gate electrode is at least included in the scan line.

The gate electrode 214 may be formed using the same material and methodas those of the source electrode 212 a and the drain electrode 212 b.

Although not illustrated, it is preferable that dopant be added to theoxide semiconductor layer 211 using the gate electrode 214 as a mask toform a source region and a drain region. The dopant may be added by anion implantation method or an ion doping method. Alternatively, thedopant may be added by performing plasma treatment in an atmosphere of agas containing the dopant. As the added dopant, a rare gas, nitrogen,phosphorus, arsenic, or the like may be used.

Note that in the case where the third etching mask is formed using aresist material, the third etching mask may be removed only by ashing.

Note that here, the dopant may be added by an ion implantation method oran ion doping method. Alternatively, the dopant may be added byperforming plasma treatment in an atmosphere of a gas containing thedopant. As the added dopant, a rare gas, nitrogen, phosphorus, arsenic,or the like may be used.

As described above, the transistor in which the channel formation regionis formed using the oxide semiconductor layer can be manufactured overthe transistor formed using the semiconductor substrate as illustratedin FIG. 6.

In addition, in the transistor manufactured as described above, theoff-state current per micrometer in channel width at room temperaturecan be lower than or equal to 10 aA/μm (1×10⁻¹⁷ A/μm), lower than orequal to 1 aA/μm (1×10⁻¹⁸ A/μm), further lower than or equal to 1 zA/μm(1×10⁻²¹ A/μm), still further lower than or equal to 1 yA/μm (1×10⁻²⁴A/μm).

Note that when a conductive film for at least one of the scan line andthe signal line is formed using copper, the wiring has low resistance,which is preferable.

The transistor described in this embodiment as a transistor in which achannel formation region is formed using an oxide semiconductor layer isan example, and without limitation thereon, various modes can beemployed for the transistor in which a channel formation region isformed using an oxide semiconductor layer.

Embodiment 2

The actually measured field-effect mobility of an insulated gatetransistor can be lower than its original mobility because of a varietyof reasons; this phenomenon occurs not only in the case of using anoxide semiconductor.

One of the reasons that reduce the mobility is a defect inside asemiconductor or a defect at an interface between the semiconductor andan insulating film. When a Levinson model is used, the field-effectmobility on the assumption that no defect exists inside thesemiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effectmobility of a semiconductor are μ₀ and μ, respectively, and a potentialbarrier (such as a grain boundary) exists in the semiconductor, themeasured field-effect mobility is expressed as a formula A in FIG. 12A.

Here, E represents the height of the potential barrier, k represents theBoltzmann constant, and T represents the absolute temperature.

When the potential barrier is assumed to be attributed to a defect, theheight of the potential barrier is expressed as a formula B in FIG. 12Baccording to the Levinson model.

Here, e represents the elementary electric charge, N represents theaverage defect density per unit area in a channel, ∈ represents thedielectric constant of the semiconductor, n represents the number ofcarriers per unit area in the channel, C_(ox) represents the capacitanceper unit area, V_(g) represents the gate voltage, and t represents thethickness of the channel.

In the case where the thickness of the semiconductor layer is less thanor equal to 30 nm, the thickness of the channel may be regarded as beingthe same as the thickness of the semiconductor layer.

The drain current I_(d) in a linear region is expressed as a formula Cin FIG. 12C.

Here, L represents the channel length and W represents the channelwidth, and L and W are each 10 μm in this case.

In addition, V_(d) represents the drain voltage.

When dividing both sides of the formula C by V_(g) and then takinglogarithms of both sides, a formula D in FIG. 12D can be obtained.

The right side of the formula C is a function of V_(g).

From the formula, it is found that the defect density N can be obtainedfrom the slope of a line in a graph that is obtained by plotting actualmeasured values with ln(I_(d)/V_(g)) as the vertical axis and 1/V_(g) asthe horizontal axis.

That is, the defect density can be evaluated from the I_(d)−V_(g)characteristics of the transistor.

The defect density N of an oxide semiconductor in which the ratio ofindium (In), tin (Sn), and zinc (Zn) is 1:1:1 is approximately1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like,μ₀ can be calculated to be 120 cm²/Vs.

The measured mobility of an In—Sn—Zn oxide including a defect isapproximately 35 cm²/Vs.

However, assuming that no defect exists inside the semiconductor and atthe interface between the semiconductor and an insulating film, themobility μ₀ of the oxide semiconductor is expected to be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scatteringat an interface between a channel and a gate insulating film affects thetransport property of the transistor. In other words, the mobility μ₁ ata position that is distance x away from the interface between thechannel and the gate insulating film can be expressed as a formula E ofFIG. 12E.

D represents the electric field in the gate direction, and B and G areconstants. B and G can be obtained from actual measurement results;according to the above measurement results, B is 4.75×10⁷ cm/s and G is10 nm (the depth to which the influence of interface scatteringreaches).

When D is increased (i.e., when the gate voltage is increased), thesecond term of the right side of the formula E is increased andaccordingly the mobility μ₁ is decreased.

Calculation results E of the mobility μ₂ of a transistor whose channelincludes an ideal oxide semiconductor without a defect inside thesemiconductor are shown in FIG. 13.

For the calculation, Sentaurus Device which is software manufactured bySynopsys, Inc. was used.

For the calculation, the band gap, the electron affinity, the relativedielectric constant, and the thickness of the oxide semiconductor wereassumed to be 2.8 eV, 4.7 eV, 15, and 15 nm, respectively.

These values were obtained by measurement of a thin film that was formedby a sputtering method.

Further, the work functions of a gate, a source, and a drain wereassumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively.

The thickness of a gate insulating film was assumed to be 100 nm, andthe relative dielectric constant thereof was assumed to be 4.1. Thechannel length and the channel width were each assumed to be 10 μm, andthe drain voltage V_(d) was assumed to be 0.1 V.

As shown by the calculation results E, the mobility has a peak ofgreater than or equal to 100 cm²/Vs at a gate voltage that is a littleover 1 V and is decreased as the gate voltage becomes higher because theinfluence of interface scattering is increased.

Note that in order to reduce interface scattering, it is desirable thata surface of the semiconductor layer be flat at the atomic level (atomiclayer flatness).

Characteristics of minute transistors which are manufactured using anoxide semiconductor having such a mobility were calculated.

The transistor used for calculation includes a channel formation regionprovided between a pair of n-type semiconductor regions in the oxidesemiconductor layer.

The calculation was performed under the condition that the resistivityof the pair of n-type semiconductor regions is 2×10⁻³ Ωcm.

The calculation was performed under the condition that the channellength was 33 nm and the channel width was 40 nm.

Further, a sidewall is provided on the side wall of the gate electrode.

The calculation was performed under the condition that part of thesemiconductor region which overlaps with the sidewall is an offsetregion.

For the calculation, Sentaurus Device which is software manufactured bySynopsys, Inc. was used.

FIGS. 14A to 14C are calculation results of the gate voltage (V_(g): apotential difference between the gate and the source) dependence of thedrain current (I_(d), solid line) and the mobility (m, dotted line) ofthe transistor.

The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage (a potential difference between the drain and thesource) is +1 V and the mobility μ is obtained by calculation under theassumption that the drain voltage is +0.1 V.

FIG. 14A shows the calculation result under the condition that thethickness of the gate insulating film is 15 nm.

FIG. 14B shows the calculation result under the condition that thethickness of the gate insulating film is 10 nm.

FIG. 14C shows the calculation result under the condition that thethickness of the gate insulating film is 5 nm.

As the gate insulating film is thinner, the drain current I_(d)(off-state current) particularly in an off state is significantlydecreased.

In contrast, there is no noticeable change in the peak value of themobility μ and the drain current I_(d) in an on state (on-statecurrent).

FIGS. 15A to 15C show the gate voltage V_(g) dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor where the offset length (the sidewall length) L_(off) is 5nm.

The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage is +1 V and the mobility μ is obtained bycalculation under the assumption that the drain voltage is +0.1 V.

FIG. 15A shows the calculation result under the condition that thethickness of the gate insulating film is 15 nm.

FIG. 15B shows the calculation result under the condition that thethickness of the gate insulating film is 10 nm.

FIG. 15C shows the calculation result under the condition that thethickness of the gate insulating film is 5 nm.

FIGS. 16A to 16C show the gate voltage dependence of the drain currentI_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor where the offset length (sidewall length) L_(off) is 15 nm.

The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage is +1 V and the mobility μ is obtained bycalculation under the assumption that the drain voltage is +0.1 V.

FIG. 16A shows the calculation result under the condition that thethickness of the gate insulating film is 15 nm.

FIG. 16B shows the calculation result under the condition that thethickness of the gate insulating film is 10 nm.

FIG. 16C shows the calculation result under the condition that thethickness of the gate insulating film is 5 nm.

In either of the structures, as the gate insulating film is thinner, theoff-state current is significantly decreased, whereas no noticeablechange arises in the peak value of the mobility μ and the on-statecurrent.

Note that the peak of the mobility μ is approximately 80 cm²/Vs in FIGS.14A to 14C, approximately 60 cm²/Vs in FIGS. 15A to 15C, andapproximately 40 cm²/Vs in FIGS. 16A to 16C; thus, the peak of themobility μ is decreased as the offset length L_(off) is increased.

Further, the same applies to the off-state current.

The on-state current is decreased as the offset length L_(off) isincreased; however, the decrease in the on-state current is much moregradual than the decrease in the off-state current.

Further, the graphs show that in either of the structures, the draincurrent exceeds 10 μA, which is required in a memory element and thelike, at a gate voltage of around 1 V. The contents of this embodimentor part thereof can be implemented in combination with any of the otherembodiments and an example.

Example 1

A transistor including an oxide semiconductor containing In, Sn, and Zncan have favorable characteristics by forming the oxide semiconductorwhile heating a substrate or by heat treatment after forming an oxidesemiconductor film.

Note that each of In, Sn, and Zn is preferably contained at greater thanor equal to 5 at. %.

By heating the substrate after forming the oxide semiconductor filmcontaining In, Sn, and Zn, the field-effect mobility of the transistorcan be improved.

The threshold voltage of an n-channel transistor can be shifted in thepositive direction.

When the threshold voltage of the n-channel transistor is shifted in thepositive direction, an absolute value of voltage for holding an offstate of the n-channel transistor can be decreased, and powerconsumption can be reduced.

Further, when the threshold voltage of the n-channel transistor isshifted in the positive direction, and the threshold voltage is greaterthan or equal to 0 V, a normally-off transistor can be formed.

Characteristics of transistors including the oxide semiconductorcontaining In, Sn, and Zn are described below.

(Common Conditions for Samples A to C)

An oxide semiconductor layer was formed over a substrate to have athickness of 15 nm under the following conditions: a target having acomposition ratio of In:Sn:Zn=1:1:1 was used; the gas flow rate wasAr/O₂=6/9 sccm; the deposition pressure was 0.4 Pa; and the depositionpower was 100 W.

Next, the oxide semiconductor layer was etched into an island shape.

Then, a tungsten layer was formed over the oxide semiconductor layer tohave a thickness of 50 nm, and was etched, so that a source electrodeand a drain electrode were formed.

Next, a silicon oxynitride (SiON) film was formed so as to have athickness of 100 nm, using a silane gas (SiH₄) and dinitrogen monoxide(N₂O) by a plasma CVD method, so that a gate insulating layer wasformed.

Then, a gate electrode was formed in the following manner: a tantalumnitride film was formed to have a thickness of 15 nm; a tungsten filmwas formed to have a thickness of 135 nm; and these films were etched.

After that, a silicon oxynitride (SiON) film with a thickness of 300 nmwas formed by a plasma CVD method and a polyimide film with a thicknessof 1.5 μm was formed, so that an interlayer insulating film was formed.

Next, a pad for measurement was formed in the following manner: acontact hole was formed in the interlayer insulating film; a firsttitanium film was formed to have a thickness of 50 nm; an aluminum filmwas formed to have a thickness of 100 nm; a second titanium film wasformed to have a thickness of 50 nm; and these films were etched.

As described above, a semiconductor device having a transistor wasformed.

(Sample A)

In Sample A, heating was not performed to the substrate during thedeposition of the oxide semiconductor layer.

Further, in Sample A, heat treatment was not performed after thedeposition of the oxide semiconductor layer before the etching of theoxide semiconductor layer.

(Sample B)

In Sample B, the oxide semiconductor layer was formed with the substrateheated at 200° C.

Further, in Sample B, heat treatment was not performed after thedeposition of the oxide semiconductor layer before the etching of theoxide semiconductor layer.

The substrate was heated while the oxide semiconductor layer was formedin order to remove hydrogen serving as a donor in the oxidesemiconductor layer.

(Sample C)

In Sample C, the oxide semiconductor layer was formed with the substrateheated at 200° C.

Further, in Sample C, heat treatment in a nitrogen atmosphere wasperformed at 650° C. for 1 hour and then heat treatment in an oxygenatmosphere was performed at 650° C. for 1 hour after the oxidesemiconductor layer was formed before the oxide semiconductor layer wasetched.

The reason why the heat treatment was performed at 650° C. in a nitrogenatmosphere for 1 hour is because hydrogen that serves as a donor in theoxide semiconductor layer is removed.

Here, oxygen is also released by the heat treatment for removinghydrogen that serves as a donor in the oxide semiconductor layer, and anoxygen vacancy which serves as a carrier in the oxide semiconductorlayer is generated.

Thus, an effect of reducing oxygen vacancies was tried to be obtained byperforming heat treatment at 650° C. in an oxygen atmosphere for 1 hour.

(Characteristics of Transistors of Samples A to C)

FIG. 17A shows initial characteristics of the transistor of Sample A.

FIG. 17B shows initial characteristics of the transistor of Sample B.

FIG. 17C shows initial characteristics of the transistor of Sample C.

The field-effect mobility of the transistor of Sample A was 18.8cm²/Vsec.

The field-effect mobility of the transistor of Sample B was 32.2cm²/Vsec.

The field-effect mobility of the transistor of Sample C was 34.5cm²/Vsec.

According to observation of cross sections of oxide semiconductor layerswhich were formed by deposition methods similar to those of Samples A toC, with a transmission electron microscope (TEM), crystallinity wasobserved in samples formed by the deposition methods similar to those ofSample B and Sample C, substrates of which were heated duringdeposition.

Further, surprisingly, the sample, the substrate of which was heatedduring deposition, had a non-crystalline portion and a crystallineportion, and the orientation of the crystalline portion was aligned inthe c-axis direction.

In a conventional polycrystal, the orientation of the crystallineportion is not aligned. Thus, it can be said that the sample, thesubstrate of which was heated during deposition, has a novel structure.

Comparison of FIG. 17A to FIG. 17C shows that heat treatment performedon the substrate during or after deposition can remove a hydrogenelement serving as a donor, so that the threshold voltage of then-channel transistor can be shifted in the positive direction.

That is, the threshold voltage of Sample B, the substrate of which wassubjected to heating during deposition, is shifted in the positivedirection more than the threshold voltage of Sample A, the substrate ofwhich was not subjected to heating during deposition.

In addition, by comparing Sample B and Sample C, substrates of whichwere heated during deposition, it is found that the threshold voltage ofSample C on which heat treatment was performed after deposition isshifted in the positive direction more than the threshold voltage ofSample B on which heat treatment was not performed after deposition.

As the temperature of the heat treatment is higher, a light element suchas hydrogen is easily removed; therefore, as the temperature of the heattreatment is higher, hydrogen is easily removed.

Accordingly, it can be considered that the threshold voltage can beshifted more in the positive direction by further increasing thetemperature of heat treatment during or after the deposition.

(Results of the Gate BT Stress Test of Sample B and Sample C)

The gate BT stress test was performed on Sample B (without heattreatment after deposition) and Sample C (with heat treatment afterdeposition).

First, V_(g)−I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(d) of 10 V to measurecharacteristics of the transistors before heating and high positivevoltage application.

Then, the substrate temperature was set to 150° C. and V_(d) was set to0.1 V.

After that, 20 V was applied as V_(g) to the gate insulating film, andwas kept for 1 hour.

Next, V_(g) was set to 0 V.

Then, V_(g)−I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(d) of 10 V, whereby thecharacteristics of the transistors after heating and high positivevoltage application were measured.

As described above, to compare the characteristics of the transistorbefore and after heating and high positive voltage application isreferred to as a positive BT test.

In a similar manner, V_(g)−I_(d) characteristics of the transistors weremeasured at a substrate temperature of 25° C. and V_(d) of 10 V tomeasure characteristics of the transistors before heating and highnegative voltage application.

Then, the substrate temperature was set to 150° C. and V_(d) was set to0.1 V.

Next, −20 V was applied as V_(g) to the gate insulating film, and waskept for 1 hour.

Next, V_(g) was set to 0 V.

Then, V_(g)−I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(d) of 10 V, whereby thecharacteristics of the transistors after heating and high negativevoltage application were measured.

As described above, to compare the characteristics of the transistorbefore and after heating and high negative voltage application isreferred to as a negative BT test.

FIG. 18A shows results of the positive BT test of Sample B, and FIG. 18Bshows results of the negative BT test of Sample B.

FIG. 19A shows results of the positive BT test of Sample C, and FIG. 19Bshows results of the negative BT test of Sample C.

The positive BT test and the negative BT test are tests used todetermine deterioration of the transistors; it is found that thethreshold voltage can be positively shifted by at least the positive BTtests with reference to FIG. 18A and FIG. 19A.

In particular, it is found in FIG. 18A that the transistor becamenormally-off when the positive BT test was performed.

Accordingly, it is found that, when the positive BT test as well as theheat treatment at the time of manufacture of the transistors wasperformed, a positive shift of the threshold voltage can be promoted anda normally-off transistor can be formed.

FIG. 20 shows a relation between the off-state current of the transistorof Sample A and the inverse of the substrate temperature (absolutetemperature) at measurement.

Here, the horizontal axis represents a value (1000/T) obtained bymultiplying an inverse of the substrate temperature at measurement by1000.

Note that the amount of current in the case where the channel width is 1μm is shown in FIG. 20.

The off-state current was less than or equal to 1×10⁻¹⁹ A when thesubstrate temperature was 125° C. (1000/T was about 2.51).

The off-state current was less than or equal to 1×10⁻²⁰ A when thesubstrate temperature was 85° C. (1000/T was about 2.79).

In other words, it is found that extremely small off-state current wasobtained as compared to a transistor including a silicon semiconductor.

The off-state current is decreased as the temperature is lower;therefore, it is clear that smaller off-state current is obtained atroom temperature.

This application is based on Japanese Patent Application serial no.2011-096622 filed with Japan Patent Office on Apr. 22, 2011, andJapanese Patent Application serial no. 2011-108900 filed with JapanPatent Office on May 14, 2011, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A semiconductor device comprising a first memorycomprising: a first transistor, a second transistor, and a thirdtransistor electrically connected to each other in series in order; anda fourth transistor, a fifth transistor, and a sixth transistorelectrically connected to each other in series in order, wherein one ofa source and a drain of the first transistor and one of a source and adrain of the fourth transistor are electrically connected to a highpower supply potential line, wherein one of a source and a drain of thethird transistor and one of a source and a drain of the sixth transistorare electrically connected to a low power supply potential line, whereina gate of the first transistor and a gate of the third transistor areelectrically connected to a third terminal, wherein the third terminalis electrically connected between the fifth transistor and the sixthtransistor, wherein a gate of the fourth transistor and a gate of thesixth transistor are electrically connected to a second terminal,wherein the second terminal is electrically connected between the secondtransistor and the third transistor, wherein a gate of the secondtransistor and a gate of the fifth transistor are electrically connectedto a first terminal, wherein each of the first transistor and the fourthtransistor is a p-channel transistor, and wherein each of the secondtransistor, the third transistor, the fifth transistor, and the sixthtransistor is a transistor comprising an oxide semiconductor layer. 2.The semiconductor device according to claim 1 further comprising: asecond memory having a structure similar to that of the first memory; aseventh transistor; and an eighth transistor, wherein one of a sourceand a drain of the seventh transistor is electrically connected to thethird terminal of the first memory, and wherein one of a source and adrain of the eighth transistor is electrically connected to the secondterminal of the first memory and the third terminal of the secondmemory.
 3. The semiconductor device according to claim 2, wherein a gateof the seventh transistor is electrically connected to a line to which aclock signal is input, wherein a gate of the eighth transistor iselectrically connected to a line to which an inversion signal of theclock signal is input, and wherein the other of the source and the drainof the seventh transistor is electrically connected to a data inputsignal line.
 4. A semiconductor device comprising a first memorycomprising: a first transistor, a second transistor, and a thirdtransistor electrically connected to each other in series in order; anda fourth transistor, a fifth transistor, and a sixth transistorelectrically connected to each other in series in order, wherein one ofa source and a drain of the first transistor and one of a source and adrain of the fourth transistor are electrically connected to a highpower supply potential line, wherein one of a source and a drain of thethird transistor and one of a source and a drain of the sixth transistorare electrically connected to a low power supply potential line, whereina gate of the first transistor and a gate of the third transistor areelectrically connected to a third terminal, wherein the third terminalis electrically connected between the fifth transistor and the sixthtransistor, wherein a gate of the fourth transistor and a gate of thesixth transistor are electrically connected to a second terminal,wherein the second terminal is electrically connected between the secondtransistor and the third transistor, wherein a gate of the secondtransistor and a gate of the fifth transistor are electrically connectedto a first terminal, wherein each of the first transistor and the fourthtransistor is a p-channel transistor, wherein each of the secondtransistor, the third transistor, the fifth transistor, and the sixthtransistor is a transistor comprising an oxide semiconductor layer, andwherein the semiconductor device is a liquid crystal display device. 5.The semiconductor device according to claim 4 further comprising: asecond memory having a structure similar to that of the first memory; aseventh transistor; and an eighth transistor, wherein one of a sourceand a drain of the seventh transistor is electrically connected to thethird terminal of the first memory, and wherein one of a source and adrain of the eighth transistor is electrically connected to the secondterminal of the first memory and the third terminal of the secondmemory.
 6. The semiconductor device according to claim 5, wherein a gateof the seventh transistor is electrically connected to a line to which aclock signal is input, wherein a gate of the eighth transistor iselectrically connected to a line to which an inversion signal of theclock signal is input, and wherein the other of the source and the drainof the seventh transistor is electrically connected to a data inputsignal line.
 7. A semiconductor device comprising a first memorycomprising: a first transistor, a second transistor, and a thirdtransistor electrically connected to each other in series in order; anda fourth transistor, a fifth transistor, and a sixth transistorelectrically connected to each other in series in order, wherein one ofa source and a drain of the first transistor and one of a source and adrain of the fourth transistor are electrically connected to a highpower supply potential line, wherein one of a source and a drain of thethird transistor and one of a source and a drain of the sixth transistorare electrically connected to a low power supply potential line, whereina gate of the first transistor and a gate of the third transistor areelectrically connected to a third terminal, wherein the third terminalis electrically connected between the fifth transistor and the sixthtransistor, wherein a gate of the fourth transistor and a gate of thesixth transistor are electrically connected to a second terminal,wherein the second terminal is electrically connected between the secondtransistor and the third transistor, wherein a gate of the secondtransistor and a gate of the fifth transistor are electrically connectedto a first terminal, wherein each of the first transistor and the fourthtransistor is a p-channel transistor, wherein each of the secondtransistor, the third transistor, the fifth transistor, and the sixthtransistor is a transistor comprising an oxide semiconductor layer, andwherein the oxide semiconductor layer comprises indium and zinc.
 8. Thesemiconductor device according to claim 7 further comprising: a secondmemory having a structure similar to that of the first memory; a seventhtransistor; and an eighth transistor, wherein one of a source and adrain of the seventh transistor is electrically connected to the thirdterminal of the first memory, and wherein one of a source and a drain ofthe eighth transistor is electrically connected to the second terminalof the first memory and the third terminal of the second memory.
 9. Thesemiconductor device according to claim 7, wherein a gate of the seventhtransistor is electrically connected to a line to which a clock signalis input, wherein a gate of the eighth transistor is electricallyconnected to a line to which an inversion signal of the clock signal isinput, and wherein the other of the source and the drain of the seventhtransistor is electrically connected to a data input signal line. 10.The semiconductor device according to claim 8, wherein the oxidesemiconductor layer further comprises gallium.